The present invention relates a computer storage system, in particular to a clustered storage system with external storage systems.
Recent computer storage systems have been provided with a function called “external connection” that connects a storage system to other storage systems. Such a function allows a storage system to discover disk volumes in the external storage systems and export them to host computers via its communication ports as if the disk volumes are internal volumes.
If the storage system has advanced functions, for example, volume mirroring, large capacity of cache memory, various host-connectivity and so on, it can provide these functions to external storage systems which do not have such functions. A storage system that has an external connection function may be referred to as a platform storage system. Storage volumes located within the platform storage system (or local storage system) are referred to as internal volumes, and storage volumes located within an external storage system are referred to as external volumes. The details of a storage system with external storage systems are disclosed in U.S. Pat. No. 6,529,976, which is incorporated by reference.
Data in the external storage system are read and written through the platform storage system. If some failure occurs in the platform storage system or at a connection link between a platform storage system and an external storage system, the data in the external storage system cannot be accessed. A conventional highly available storage system which duplicates data in two storage devices, e.g., in two different disk array units, can be used to ensure that the data are accessible even when the platform storage system fails; however, such a system is very expensive.